Automatic multichannel selection



June 9, 1953 D. F; HQTH EIAL 2,641,757

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kok UNHQW INVENTORS 5 R. O. SOFFEL v Gt ATTORNEY June 9, 1953 D. F.HOTH" Au'rom'rxc sma'cuon Filed May '17. 1950 55 3 39 39 N99 89 m GtATTORNEY June 9, 1953 D. F. HOTH' EnAL 2,641,757

AUTOMATIC MULTICHANNEL- SELECTION Filed May 17. 1950 9 Sheets-Sheet 5007 ,aoa

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ATTORNEY June 9, 1953 Filed May 17. 1950 D. F. HOTH ETAL' AUTOMATICMULTICHANNEL SELECTION 9 Sheets-Sheet. 7

ATTORNEY June 9, 1953 D. F. HOTH sr-AL,

AUTOMATIC mumxcnmm. SELECTION 9 Sheets-Sheet 8 Filed May 17, 1950 0. EHOTH R0. SOFFEL IN l E N TORS QUE 25 :5 new. may 69 ATTORNEY June 9,1953 D. mom ET AL-- I 4 AUTOMATIC MULTICHANNEL SELECTION Filed May 17,1950 QSfieeis-Sheet 9 101.5 TONE nei'scron CONTROL CIRCUIT MOB/L E 8 TAT/ON DJ? HOTH R0. OFFEL A 7' TORNE V /N l/EN TORS FIG. /0

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Patented June 9, 1953 AUTOMATIC MULTIOHANN EL SELECTION Daniel F. Hoth,Millburn, N. 3., and "Robert 0.

Sofiel, Hastings on Hudson, N. Y., assignors to 1 Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication May 17, 1950,;se'ria1 a16a516 f I This invention relates totelephone systems and especially to a multichannel common mediumtransmissionsystem between a central or base station and a plurality ofmobile stations.

An object of the invention is to enable any one of a. large number ofoutlying stations to be selectivelysignaled from a central calling.station and bedirected to connect itself to a particular one of aplurality of transmission channels, and to deny all other stationsaccess to this channel.

Another object of the invention is to enable any one of a large numberof outlying stations to. establish a connection to the base station overany idle one of a plurality of transmission channels, and to cause allother stations to be denied access. to this channel.

Although this invention may be employed in any communication orsignaling system, the embodiment in which the invention is heredescribed is a. mobile radio telephone system. As specifically appliedto such a system, one of the objects is to provide a greatly improvedgrade of service to mobile radio subscribers. Heretofore, mobile radiosystems have operated on the same principle as a. party line except thatthe number of mobile, subscribers greatly exceeds the number of partiesthat would ordinarily be put on one line. By providing each mobilesubscriber with accessto a plurality of channels, the grade of service.so obtained is much better than that obtained by dividing the number ofmobile subscribers insmaller groups and assigning each group toone.channel only. Furthermore, a larger number of mobile subscribers can beaccommodated by a multichannel system where all mobile subscribers haveaccess to all channels than by a system having several exclusive chan-Another object of the invention is to provide privacy for the mobilesubscribers by denying access of other subscribers to busy channels Inthis system each mobile station has access to a plurality ofcommunication channels and to a common signaling channel. When'themobile station is not in use its receiver is always tuned to thefrequency of the common signaling channel. Each base station transmitterradiates a modulation of distinctive frequency, hereinafter called theidle tone, Whenever the channel associated with that transmitter isnotin use, and the mobile station can seize only a channe marked withthis idle tone. 7

In this specification a transmission channel will be understood tocomprise two carrierfrequencies, one for transmission in one directionand 6 Claims. (01. 34.34 77;

. tion. The signaling'or calling channel, however,

theiotherforj transmission in the opposite direcusesonly one-frequencyand hence may be considered only half a channel. At the base station aseparate transmitter andreceiver are provided for each communicationchannel and a separate transmitter for the signaling channel. Eachmobile station is providedwith atransmitter and a receiver.

Although for the purpos'e'of this invention any system of selectivesignaling may be employed, the method herein described comprises thesimultaneous transmission of a combination of any four of thirty allotedaudio frequencies, each mobile station being assigned an exclusivecombination, as described in the copending applications of C. Harrison,Serial No. 767,487, filed August 8,1947, and D. F. Hoth-R; O. Soffel,

Serial No; 56,186, filed October'23, 1948, the former having beenissued'on July 8, 1952 as Patent 2,602,853 and'the latter having becomeabandoned.

The invention will be better understood from the following detaileddescription together with the accompanyingv drawings, in which:

Fig. l is'a block diagram indicating the order I the base station;

Fig. 9 shows the combiner circuit of the base station and transmittersand receivers of the base station and of a mobile station; and

Fig. 10 shows details of the control circuit of a mobile station.

Figs. 3 to 10 inclusive, when juxtaposed in the order shown in Fig. '1,illustrate the complete system. I I

The general operation of the invention will first be explained in termsof theblock diagram shown'in Fig. 2. In this diagram, as well as in thesucceeding figures, a system comprising three communication channels isillustrated, but it will be understood that by extension of the circuitsdescribed a-larger number of channels may; be provided for.

Referring-to Fig. 2; when the operator at the base station desires tooriginate a call to a mobile station she will insert the plug of hercord circuit in a calling jack, as for example jack I, provided thischannel is idle. idle tone to be removed from this channel, thuspreventing any mobile station from originating a call on this channel,as will be explained herenected to the translator and combiner 4,provided no other line circuit is attempting to originate a call at thesame time.

, the four-digit call number into a combination of four out of thirtyaudio frequencies in a man- ,ner disclosed by the aforesaid copendingapplication ofHoth-Sofiel, Serial No. 56,186. These frequencies arecombined and transmitted over lead group 2I to the signaling channeltransmit-. ter to modulate its carrier frequency and cause the latter tobe radiated. This signal is received by all idle mobile receivers, butnot more than one of them can respond.

' When the register 3 is connected to the translator and combiner 4 analternating current of distinct frequency, hereinafter called the stoptone,'is caused to be radiated by the transmitter I of'channel I.

The calling signal transmitted over the calling channel by the basestation is received by the desired mobile receiver represented by block9 of Fig. 2A, and is transmitted over lead group 22 to the controlcircuit represented by block Ill. The control circuit, 'on reception ofthe calling signal, causes the tuning control represented by block II tooperate over lead 26. The tuning control, operating over lead groups 25and 21, causes tuning mechanisms in the receiver 9 and the transmitter I4 to hunt over each channel setting in turnp When the channel is reachedon which the stop tone radiated by the base station is being received,the control circuit I0 causes the tuning control I I to stop hunting andthe call bell I2 to ring over lead 30. When the subscriber removes thehandset I3 from the switchhook the transmitter I4 is activated over lead26 and radiates carrier. The output of the mobile receiver 9 isconnected over lead group 22, through the control circuit I0, and overlead group 23 to the handset I3. The handset is also connected over leadgroup 29 to the transmitter IA. The mobile station is now in the talkingcondition. When the base station receiver 8 receives carrier from themobile station the codanv Whena mobile subscriber originates a call byremoving his handset I3 from the switchhook, the control circuit Inoperates the tuning'control II over lead group 26, causing the receiver"9 to hunt for an idle channel. At the same time the tuning control II,over lead group 21, causes the mobile transmitter I I to hunt over thechan- This will cause the- The translator converts nels in synchronismwith the receiver. When the receiver finds a channel on which idle toneis present, hunting stops and the mobile transmitter I3 is energizedover lead group 28 and consequently radiates carrier energy having amean frequency corresponding to one of the two frequencies assigned tothe idle channel.

When the particular base station receiver that is tuned to theabove-mentioned idle channel receives the carrier energy radiated fromthe ,mobile transmitter, it energizes its codan relay thereby causing(a) immediate removal of idle tone from this channel, and (b) a linelamp to light to indicate to the operator that a mobile station hasoriginated a call.

Call originated by base station A detailed description of the inventionwill now be given for the case oia call originating at a base station.

Referring to Fig. 3, 'when'the operator desires to originate a call sheinserts the plug of her cord circuit into the jack of an idle line, asfor example jack 3I, which will be designated channel No. I. Insertionof the plug into this jack connects ground through the armature andspring No. I of jack 3|, through the contact 5' and armature of relay3'! and through the winding of the tone control relay 38 to battery,causing the latter relay to operate. The operation of relay 38 openscontacts I and 4, disconnecting the idle tone oscillator 46 which hadpreviously been connected to the transmitter 12, Fig. 9, for thischannel, over the'following path: From the idle tone oscillator'MIthrough contact I and armature of the tone control relay 38, throughcontact I and armature of the codan operated relay 39; over conductorTCI which together with conductor TCZ constitutes a tone circuit TC,

through contact 4 and armature of the talk relay 3'I, over lead 302 tothe transmitter 'I2in' group of six relays similar to relays TI to 82,com-- prise a dial pulse counter, steering and register. circuit'similarto that described in detail in the aforesaid copending application ofHoth-Soffel;

Serial No. 56,186. When the operator has completed dialing four digits,relays I4 and 15 in Fig. 6 for the thousands and hundreds digits,together with similar relays for the'tens and units digits in block 76of Fig. 6, are all operated. A path is then closed from ground on thearmature of relay I4 through-its contact I, through the armatureandcontact I of relay I5 and over lead 60I to block 16, through similarcontacts on relays for the tens and units digits, over lead 602 to Fig.3, through the armature and contact 4 of the unoperated relay 39,through the armature and contact 5 of the operated relay 38, over lead 3to Fig. 4 and through the Winding of the first start relay II tobattery, causing the latter to operate. contact network comprising theupper contacts of the start relays GI, 42 and 43 is used to indicatewhether one or more than one start relay is energized. The operation ofrelay 4| connects ground on its armature to its contact 2, through Thethe armature and contact '3 of the. unoperated intermediate start relay.42, through: the. armaand contact 4 of the un pera ied relay 43...toterminal 3 ofarc 56. of the sel or. Therefore. i the selector happ ns tobe standing on: terminal .Z, for example, representing he correspondingline and channel, ground is applied over the Wiper of are 56. throu hthe arma ur and contact- I of selector operating magnet 52, and throughthe windin of the selector control relay*4 a to battery. ca ng; hisrelay to. operat The. operate tion of relay 43 connects round from itsarmature to contact 3, and throughv th winding of the selector magnet 52to battery, causing the latter to operate. The operation of the selectormagnet opens the circuit to relay 48, causing it to release. The releaseof relay 48 removes ground from the selector magnet winding, causing itto release. The release of the selector magnet 52 mechanically advancesthe selector I22 to the next position. In the present case this would beposition 3, where ground is again connected to the wiper of are 56,causing the-above cycle to be repeated. In this manner the selectorwillbe advanced until it reaches position 1, corresponding to the linecircuit into which the operator has plugged. With relay 4| operated,ground is removed from contact I of are 56, preventing the furtheroperation of the selector control relay 48. I

When relay 48 operated, as previously described, it connected groundfrom its armature to its contact 2 through the winding of the startringing relay 4'! to battery, causing the latter relay to operate. Theoperation of this relay performs no. function at this time. Relay 41,being slow to release, holds operated during the time the selector ishunting for the channel having its start relay operated. When theselector stop hunting, relay 48 remainsv released, and after an intervalof time relay 4'! releases. The release of relay 4,! closes a path fromground onv the armature and contact I of the operated relay 45 throughthe armature and contact I of relay 41, through the armature and contactI of the unoperated relay 49, and through the winding of the stop tonerelay 50 to battery, causing the latter to operate. The operation ofrelay 50 connects lead 404 from one sideof'the stop tone generator 5|,through the armature and contact I of relay 50, through the wiper andcontact I of arc 53 of the selector, to lead 3I0 to Fig. 3, through theNo. 3 contact and armature of the operated relay 38, through the No. 2contact and armature of the unoperated relay 39, through contact No. Iand armature of the unoperated talk relay 31, and over lead I to thetransmitter I2 in Fig. 9; and back from the transmitter over lead 302 toFig. 3, through the armature and contact No. 4 of relay 37, through thearmature and contact No. I of relay 39, through the armature and contactNo. 2 of operated relay 38, over lead 309 to Fig. 4, through contact No.I and wiper of are 54 of the selector,

. Hoth-Soffel, Serial No. 56,186.

through; contact No. "2 and armature. of the operatedrelay 50, andoverlead. 405, to. the. othensicle of the stop tone generator 5I. c

When relay 45,- operated; as; previously described, it: connectedgroundfrom'its armature through its contact I over lead 308 to. Fig; .9to energize the transmitter-control circuit 63', which enabledthesignaling-z channel transmitter 68,

When the selector I22 (Fig. 4) stopped on positionv 1, ground'was.connected fromthe armature andcontact I of the operated relay 45 throughthe armature. and contact I of the releasedrelay 41, through the.armature-and contact 2: of the'unoperated relay 44, through the wiperand contact I of arc v55 of the selector, through contact 4 and armatureof the. operated relay 4|, over lead 3.0.!

to. Fig.- 7, and through the winding of the connector relay 51; tobattery, causing the. .latterto operate. .The operation of relay 51connects ground from the No. I contacts of Whichever rear dreds digit.The contact network shown on relays 83 to 94 in Fig. 8 is used totranslatethe thousands and hundreds digits of a. four-digit numberinto acombination'of two out of fifteen frequencies (ii to fl5, Fig. '8) inthe manner described in the aforesaidcopending application of Block 95comprises a similar group of relays for translating the tens and unitsdigits. The four frequencies. se-

lected by the translator are connected over leads- Bill, 802, 803 and804 to Fig. 9, Where they are amplified and combined by means ofamplifiers BI, 62, G4 and 65 and hybrid coils 63, fifiand 61'. Theoutput of hybrid coil '61 is connected to the input of the signalingchannel transmitter 68. modulating its carrier. These tones are now.received by all idle mobile receivers. Referring to Fig. 9, thereceivers 9-7 of all idle mobile. stations are tuned to'the callingchannel in thestandby condition. Each receiver is of the commonsuper-heterodyne type in which the signal froma local oscillator iscombined with the incoming signal to produce an: intermediate frequencysignal. By changing theirequency of the local oscillator the receivercan be tuned to. a different channel. -In the specific embodimentdescribed herein,-a crystal-controlled local oscillator is used and thefrequency of the oscillator is changed'by substituting a crystal tunedto an.- other frequency. However, any receiver capable of being tuned todifferent frequencies may be used if suitable tuning arrangements arepro.- vided. In Fig. 9, crystal I09 tunes'the receiver to thesignalingchannel and crystal. Il0itocommunication channel I. The four signalingtones radiated by the base station are'r'eceived by each idlemobilereceiver 9-! and are transmitted over lead I. to the selectorcircuit 98 shown in Fig. 10, and back overv lead 902 to the mobilereceiver. The signaling tones operate the selector of the desired mobilestation, and of no other, in the manner described in the pendingapplication of Hoth-Soifel, Serial No. 56,186, thereby operatingtheselector relay 99. Operation of the selector relay applies groundthrough its contact I andarmature 'to the coil. of

thesignaling lock relay I and thence to battery,

thus operating the signaling lock relay. Prior to the operation of thesignaling lock relay, the output of the mobilereceiver 91 shown in Fig.9 was connected over lead 99I through the armature and contact I of theunoperated signaling lock relay I00 to the input of the idle tonedetector I0 I, and back over lead 902 to the mobile receiver. Operationof the signaling lock relay I00 transferred the receiver output from theidle tone detector IOI to the input of the stop tone detector I02 bytransferring lead 90I from contact I to contact 2 of the signaling lockrelay. A path is established from ground through contact I of theoperated selector relay 99, through the armature andcontact 3 of theoperated signaling lock relay I00; through contacts I and 2 of theunoperated stop tone relay I03, through the armature and contact 3 ofthe stop tone relay, through contact 'I and the'armature of theunoperated switchhook 404, over lead IO0I to the motor I05 shown in Fig.9, and'thence to battery. The motor starts running, driving the tuningswitches I06, I01 and I08. When the Wiper of switch I0'I moves ashortdistance, ground is applied to lead 903. The continues to operate andthe w igr/of itch I08 moves off 'SQWOBIIBCfiHg crystal I09 from the moeceiver 91. The four signaling tones are no longer received and theselector relay 99, Fig. 10, releases. The signaling lock relay I00remains operated by a path from battery through its coil, through itsarmature and contact 3, through contacts I and 2 of the unoperated stoptone relay I03, over lead 903 to the wiper of switch I0'I, Fig. 9, andthence to ground. The motor I05 continues to operate over a path fromground through switch I0'I, lead 903, the armature and contact 3 of theunoperated stop tone relay I03, through contact I and the armature ofthe unop- .erated switchhook I00, and thence over lead IO0I to thewinding of the motor I05, Fig. 9, and thence to battery. The wiper ofswitch I08 moves to segment 2 thereby connecting crystal I I0 to themobile receiver 91 and thus efiecting the tuning of the receiver 91 tochannel I. Since stop tone is being transmitted from the base station onchannel I, it is received and transmitted over lead 90I through armatureand contact 2 of the operated signaling lock relay I00, Fig. 10, to theinput of the stop tone detector I02 and back to the receiver over lead902. The signal is transmitted through the band-pass filter III and therectifier II2 to coil I of the stop tone relay I03. The stop tone relayoperates and closes a path from. ground through its armature and itscontact 4 to the call -bell I I3 and thence to battery thereby causingthe .call bell to ring. The signaling lock relay I00 releases whencontacts I and 2 of the stop tone relay I03 open. The motor I05, Fig. 9,stops when contact 3 of the stop tone relay I03, Fig. 10, opens. Thereceiver remains tuned to. channel I. Since switch I06, Fig. 9, has alsobeen driven around by the motor, crystal I I9 is now connected to themobile transmitter I I! and the transmitter is tuned to channel I.

Again referring to Fig. 10, a path is established from ground throughcontact 5 and the armature of the operated stop tone relay I03 to thecoil of the control relay I I4 and thence to battery, operatingthe'control relay.. When the signaling lock relay I00 is released'by theoperation of the stop tone relay I03, as described above, therebydisconnecting the output of receiver 91 from the stop tone detector I02,relay I03 remains operated by Ta path from battery through its 00112,through tore 'contact12 and the armature of the unoperated switchhookI04, through the armature and contact I of' the unoperated idle tonerelay II5, through'the armature and contact 2 of the operated stop tonerelay I03, over lead 903 to Fig. 10 Y 'to switch I01 and thencetoground.

armature and contact 4 of the switchhook I04 and thence to ground. Thetransmitter thereupon radiates carrier. Operation of switchhook I04releases relay I03 and locks relay 4 by a path from battery through thelatters coil, through its contact I and armature, and through contact 3and the armature of switchhook I04 to ground. The switchhook I00 is soadjusted that contac't'3 makes with its armature before contact 2 breaksfromits armature. The telephone receiver I of the handset -I I0 isconnected through contact 3 and the armature of the operated controlrelay H4, through contact 5 and the armature'of the operated switchhookI04, thence over lead I to the mobile receiver and back over lead 902 tothe telephone receiver. The mobile station is now in the talkingcondition. All relays in Fig. 10 are unoperated except the control relayH4, which remains operated for the duration of the call.

When the base station receiver I0 (Fig. 9) receives carrier from themobile transmitter, its codan circuit 'II is caused to connect groundover lead 305 to Fig. 3, through the winding of the codan operated relay39 to battery, causing this relay to operate. The operation of relay 39opens contacts I and 2, removing stop tone from the transmitter I2 ofchannel I in Fig. 9. The operation of relay 39 also connects ground fromits armature and contact 3 through the line lamp 35 to battery, causingthe latter to light. When the line lamp operates the operator releasesdial key 33, causing the register to restore to normal. The operation ofrelay 39 opens the circuit through its No. 4 contact, causing therelease of the first start relay ll in Fig. 4. The release of relay 4|opens the circuit to relay 05, allowing it to be released. The releaseof relay 45 restores all relays in Fig. 4 to normal, releases theconnector relay 51in Fig. 7 and restores the signal channel transmitter68 in Fig. 9 .to normal.

To establish the talking path, the operator operates the talk key 32 inFig. 3, connecting ground through its contact to the winding of the talkrelay-31, causing it .to operate. The operation of relay 3! disconnectsthe conductors TCI and T02 of the tone circuit TC from leads 302 and 30I, respectively, and immediately thereafter connects the transmitter andreceiver in Fig. 9 to the hybrid coil 36 over lead 30I to 304.

When the called subscriber returns his handset IIB to the switchhookI04, Fig. 10, switchhook contact 4 and the-armature open, therebyopening the path to the transmitter control circuit I I8, Fig. 9, andturning off the mobile transmitter I IT. The armature and contact 3 ofthe switchhook open, releasing the control relay II4. A path isestablished from ground through the arc and wiper of switch I0'I, Fig.9, over lead 903 to the armature and contact 3 of the unoperated stoptone relay I03, Fig. 10, through contact I and the armature. of theunoperated switchhook I04,

thence over lead IO0I to the motor I05, Fig. 9,

11? drives switches I06, I01 and I08. Switch I08 connects crystal H inplace of I09, tuning the mobile receiver 91 to channel I. Assumingchannel I is idle, idle tone is being transmitted on channel I by thebase station. The idle tone is transmitted from the mobile receiver 91over lead 90I, through the armature and contact I of the unoperatedsignaling lock relay I00, Fig. 10, to the input of the idle tonedetector IOI and thence back over lead 902 to the mobile receiver. Inthe idle tone detector IOI the signal is transmitted through theband-pass filter I20 and the rectifier I2I to the coil of the idle tonerelay I I5, causing it to operate. A path is established from groundthrough contact 2 of the idle tone relay and its armature to the coil ofthe unoperated control relay I I4 and thence to battery, causing thecontrol relay to operate. When the idle tone is subsequently removed atthe base station, the control relay II4 continues to operate through apath from battery, through its winding, through its contact I andarmature, through contact 3 and the armature of the. operated switchhookI04 and thence to ground. The motor I05, Fig. 9, stops when itsoperating path is opened by the opening of contact 4 and the armature ofthe control relay II4, Fig. 10. A path is established from groundthrough contact 4 and the armature of the operated switch hook I04, Fig.10, thence through the armature and contact 2 of the operated controlrelay I I4, and thence over lead I002 to the transmitter control circuitI I8, Fig. 9, causing the transmitter control circuit to activate themobile transmitter Ill. The output of the mobile receiver is connectedover lead 90I through the armature and contact 5 of the operatedswitchhook I04, Fig. 10, through the armature and contact 3 of theoperated control relay II4, thence to the telephone receiver I of thehandset H0, and back over lead 902 to the mobile receiver. Thetransmitter 2 of the handset H6 is connected over lead I003 to the inputof the mobile transmitter ,I N, Fig. 9, and back over lead I004 to thetelephone transmitter of the handset. The mobile station is now in thetalking condition.

When the mobile carrier is received by the base station receiver 10 inFig. 9, its codan circuit II is energized, connecting ground over lead305 to Fig. 3, causing the codan operated relay 39 to operate.Operationof relay 39 disconnects the idle tone oscillator from theconductors TCI and TCZ of the tone circuit TC connected by contacts Iand 4 and associated armatures of relay 31 to the base stationtransmitter I2 in Fig. 9 for this channel. Operation of relay 39connects ground from its armature and contact 3 to line lamp 35, causingit to light. When the line' lamp lights, the operator plugs the cordfrom her cord circuit into the line jack SI, operating relay 38 asdescribed for a base station originating call. Relay 38 at this timeperforms no useful function. The operator then operates talking key '32which connects ground through its contact to operate talk relay 3?,which connects the base station transmitter and receiver to the hybridcoil 36 as previously described. The operator can now determine whatnumber the mobile subscriber desires and establish the call in thestandard manner. At the endof the call, disconnection takes place aspreviously described for a base station originating call.

If the mobile station attempts to originate a call when no channel isidle, the operations are the same as described previously for a mobileabandons the attempt, whereupon it stops hunting and the mobile receiveris again tuned to the I calling channel.

It is to be understood that the above-described arrangement isillustrative of the principles of the invention. Other 'arrangementsmaybe devised by those skilled in the art without departing from the spiritand scope of the invention.

What is claimed is: I

1. In a two-way. radio communication system including a base stationhaving a plurality of transmitters and receivers all tuned to differentfrequencies, each of said transmitters being paired with a respectivelydiiferent one of said receivers, the circuits and carriers associatedwith each pair of said transmitters and receivers constituting acommunication channel, a separate calling transmitter at said basestation tuned to still another frequency, and means for adding a controltone designated idle tone on the carrier of each idle communicationchannel, and a plurality of mobile stations each having a tunabletransmitter and receiver, tuning means for tuning said mobiletransmitter and receiver successively to said communication channels orto said calling frequency, said mobile receivers in the standbycondition being tuned to said calling frequency, a telephone instrument,holding means for holding said instrument during idle periods, and acall indicator;

means under the control of said base station operator for tuning thetransmitter and receiver of a desired mobile station to a selected idlecommunication channel comprising means at said base station for adding acombination of selec-' means, and means controlled by said stop tonedetector for stopping the operation of said tuning means so that saidtransmitter and receiver remain tuned to the said selected channel,means controlled by-said calling tone detector for operating said callindicator, and means at said base station responsive to waves emitted bysaid tuned mobile transmitter. for removing said stop tone from saidselected channel; means responsive to the removal of said instrumentfrom said holding means upon initiating -:a call from said mobilestation to start the operation of said tuning means, a selectivedetector of said idle tone in the output of said mobile receiver, meanscontrolled by said idle tone detector for stopping the operation of saidtuning means so that said transmitter and receiver remain tuned for theduration of the call to the first channel marked by saidid'le tone thatis tuned in, means at said base said mobile stations from tuning to thesame. channel, and meansat said mobile station for. automaticallyretuningzsaidimobile receiver to 9 and thence to battery, causing themotor to run. The telephone receiver I of the handset I I6 is dis.-

connected from the output of the mobile receiver 91, Fig. 9, by theopening of contact and the armature of the switchhook I9 5, Fig. 10. Themotor continues to run until the signaling chan-. nel is reached,whereupon switch IG'I removes ground from lead 903, causing the motor tostop. When the mobile carrier is removed, the coclan circuit II in Fig.9 is restored to normal, allowing relay 39 in Fig. 3 to release. Therelease of relay 39 extinguishes the line lamp 35. The operator thenrestores the talking key to normal and re.- moves the plug from the linejack 3|, whereupon all relays in Fig. 3 restore to normal.

M ObiZe stati foils t0 (UL-$ 2? A case will now be described in whichthe. mos bile. subscriber fails to answer a call from the base station.The circuits perform as described above until he. Stop tone relay 583 ofFig. 10 has operated and the call bell I I3 rings. At this point thestoptone relay I03 is operated and all other relays of Fig. 10 areunoperated. The tuning mo, tor I615, Fig. 9, is stopped. If, after asuitable interval, the operator at the base station abandons the attemptto reach the mobile station, she removes her plug from line jack 3I,Fig. 3. This causes all relays in the base station to restore to normaland idle tone to be restored to the .chane nel. Idle tone is received inthe mobile receiver, Fig. .9. The idle tone is transmitted over leadsill, throu h the armature and. Contact I of the unoperated signalinglock relay I99. Fig. 10. to the i le. tone detector I GI and back overlead 9.32 to. the mobile receiver. In the idle tone detector,

the idle tone passes through the band-pass filter" ontact I and the amature of the. switchhook I84.

Simultaneous calls by base station A description will now be given ofthe case in which wo out oin calls are originated simultan usly from theba e s a on Assumin these calls to originate on channels I and 2 in themanner previously described, the first, start relay 4|,

in Fig. 4 will be operated. In a similar manner the intermediate startrelay 42 will also be operated. The'path now established from ground onthe armature of the operated relay 4| through its No. 2 contact, throughthe armature and No. 4 contact" of the operated relay 42, and throughthe winding of the multiple start relay 44 to battery, causes the latterto operate. The operation of relay 44 sets in motion the same chain ofevents as previously described for the operation of relay 45. Theselector will advance until it reaches either terminal l or 2 of theselector. When the selector stops, relay 4'! releases as preringingtiming relay .46, through the wiper and contact I of arc 550i theselector, and over the path previously described to operate theconnector relay 51in Fig; '7. Y

When the selector control relay 48 (Fig. 4) re.- leased at the time theselector stopped hunting, a path was closed from the armature andcontact I of the operated relay #4, through the contact I and armatureof relay 48., and through the winding of relay 46 to battery. Relay 46is slow to opcrate, requiring approximately one second. Afterapproximately one second, relay 46 operates, opening the circuitpreviously described to the connector relay 51 in Fig. '7. This removesthe ringing tone from the signaling channel transmitter. The operationof relay 46 connects ground from its armature and No. -I contact throughthe armature and No. 3 contact of the operated relay 44 to operate thehunting timing relay 49. This relay is slow to operate in order to allowthe mobile receiver time in which to find channel I if it has alreadystarted to hunt. After an interval sufiiciently long to insure that allmobile receivers are again on the signaling channel, relay 49 operates.The operation of relay 49 opens the circuit to the winding of the stoptone relay 56, causing it to release and remove stop tone from channelI. also connects ground from its armature through its No. 2 contact tothe winding of the selector control relay 53, causing it to operate. Theoperation of relay 48 energizes selector magnet 52 as previouslydescribed. The operation of relay 43 also opens the circuit to theringing timing relay 46, causing it to release. The release of relay 36opens the circuit to the winding of relay 49-, causing it to release.The release of relay 49 permits relay 48 to release, in turn releasingselectormagnet 52 and allowing the selector to advance to position 2.Since the intermediate start relay 42 is also operated, the selectorremains in this position-and a path similar to the one previouslydescribed for channel I is established to operate the connector relay inthechannel 2 register and to connect stop tone to channel 2. If thesubscriber being called on channel 2 does not respond within the timeprovided by the ringing timing relay 46, the selector is again caused tohunt. In the case being described the selector will hunt to position 1.The above sequence of events will continue until one of the subscribersanswers, whereupon the corresponding start relay will be released aspreviously described. The release of this start relay will cause therelease of the mul-. tiple start relay 44 and the operation of thesingle start relay 45. Ringing will now continue on this channel ntilthis sub-scriber answers. The cir- Quits are restored to normal asdescribed above.

Call originated by a mobile station A mobile originating call will nowbe described. Referring to Fig. 10, the subscriber removes his handsetII6 from the switchhook H34. Operation of the switchhook contacts causesthe motor I65, Fig. 9, to operate through a path from battery throughits windings, through lead IEIIII to the armature and contact 6 of theoperated switchhook, and through the armature and contact 4 of theunoperated control relay H4. The motor The operation of relay d9 saidcalling frequency upon the termination of a call.

2. The combination specified in claim 1 wherein said mobile stationtuning means comprises tunin crystals for said transmitter and receiver,a selecting switch for said crystals, and a motor for operating saidswitch.

3. A radiant energy signaling system having a plurality of radiantenergy signaling channels allotted thereto for the transmission ofcarrier waves, said system including a signaling station comprising incombination a plurality of transmitting means each operable to radiatecarrier energy over a respectively different one of said channelscontinuously, a plurality of line circuits each extendin to arespectively different one of said transmitting means, each of said linecircuits including a tone circuit having a source of idle tone normallyconnected thereover to the associated transmitting-means for modulatingits carrier energy, a single source of stop tone, a plurality ofconnecting circuits extending from said stop tone source to each of saidline circuits, each of said line circuits having control means forconditioning it for the transmission of intelligence signals to theassociated transmitting means for modulation of its carrier energy, andelectroresponsive means in each of said line circuits responsive to theoperation of the associated control means for disconnecting theassociated idle tone source from the associated tone circuit and forsubstituting therefor said stop tone source.

4. A radiant energy signaling system in accordance with claim 3 andhaving a second signaling station comprising in combination radiantenergy communication equipment including a telephone instrument andholding means for holding said instrument during idle periods, tuningmeans including motor-driven means for tuning said equipment foroperation with any one of said channels, and operating means responsiveto the removal of said instrument from said holding means for operatingsaid motor-driven means to tune said equipment to one of the channelsover 1 which said idle tone modulated carrier is being transmitted. V

5. A radiant energy signaling system in accordance with claim 3 whereinsaid signaling system has a calling channel allotted thereto and whereinsaid signaling station includes additional transmitting means forradiating carrier energy continuously over said calling channel, saidstop tone source being normally disconnected from said line circuits,calling means in each of said line circuits for impressing callingsignals upon the carrier transmitted over said calling channel, andadditional electroresponsive means responsive to the operation of thecalling means in any one of said line circuits for connecting said stoptone source to a selected one of said connecting circuits for modulationof the carrier energy of the associated transmitting means.

6. A radiant energy signaling system in accordance with claim' 5 andhaving a second signaling station comprising in combination radiantenergy communication equipment, tuning means including motor-drivenmeans for tuning said equipment for operation with one of said channels,a portion of said equipment being normally tuned to said callingchannel, and operating means responsive to the reception by saidequipment of an assigned combination or" calling signals transmittedover said calling channel for operating said motor-driven means to tunesaid equipment to the channel over which said stop tone modulatedcarrier is being transmitted.

- DANIEL F. HOTH.

ROBERT O. SOFFEL.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,608,551 Demarest et a1. Nov. 30, 1926 2,265,056 Bowers Dec.2, 1941 2,383,908 Bowers Aug. 28, 1945 2,458,558 Bradley Jan. 11, 19492,475,675 Peterson July 12, 1949 2,479,701 Ress Aug. 23, 1949 2,501,091Preston et a1 Mar. 21, 1950

